Trailing locomotives in a multiple-unit consist are controlled from the lead unit by means of a known system using twenty seven wires extending through each locomotive unit. Connections are made to each adjacent unit through a twenty seven conductor jumper cable. Two of the wires used for directional setup are designated as forward and reverse. These are swapped at one end of the locomotive and swapped back to normal in the jumper cable. With this arrangement, any locomotive oriented in the reverse position will receive its forward or reverse signal in the opposite wire of the pair, and thus establish a direction of travel identical with the lead locomotive.
Slave locomotives, situated elsewhere in the train, are connected to each other in the above manner but must receive their commands from the controlling unit by radio. Even with error-free communication, the forward and reverse commands will arrive at the slave consist with ambiguous meaning because of the unknown orientation of these units relative to the controlling unit. Presently the slave consist must be set up manually to provide a directional reference.
The rail industry is now taking interest in the use of an electrically controlled pneumatic (ECP) train brake system. In its present form ECP employs a two-conductor cable trainline extending through the length of each car and locomotive for transmission of both digital control signals and DC power from the controlling locomotive to energize the electric devices on each car. The Association of American Railroads (AAR) has generally specified a standard inter-car connector for this purpose consisting of both a male and female component, such that, trainline connectors on adjacent cars will mate properly regardless of car orientation. An effort is ongoing to expand the ECP specification whereby the two-conductor trainline cable can serve as a platform for control of slave locomotives in lieu of radio. If fully implemented, this method could supplant the present twenty seven conductor trainline in all locomotives including those in the head-end consist. However, this would require an alternative directional reference for trailing units at least as automatic and foolproof as the present swapped wire arrangement.